Sunday, September 19, 2010

Transparent Aluminum

Remember in Star Trek IV when Scotty gives that dude the recipe for transparent aluminum? Well, I just found out that clear aluminum exists. Life modeling art, perhaps? I wonder sometimes…

I think that Star Trek and other sci fi classics have been giving nerds with know-how ideas for new technology for years. The flip phone for example...tell me that wasn't inspired by Kirk's communicator. Sliding doors, like the kind they have in the grocery store: Star Trek all the way.

Anyway, clear aluminum! How cool. Imagine what a strong window you can make, with really thin panes! Or if you could see your canned food from the outside, what would that be like? No more shaking it and making a guess when the label has been peeled off. Clear aluminum would certainly make dumpster diving easier. And clear aluminum foil?? HOW COOL. Just sayin.

So now I'm about to nerd out on transparent aluminum. I have to start with why things are transparent in the first place.

Ok, let's start with glass. Why is glass see through? It's no news that glass is made from sand, which is a silicate. They melt it down and the individual molecules start moving in random directions because that's what happens when you heat stuff.

The special thing about most solids is that they have a crystalline structure, meaning all the constituents, be they atoms or molecules, form a 3D repeating pattern. Think about a cube that you made out of sticks and balls of clay or something. The balls of clay are the corners. You can poke more sticks into the clay and keep this pattern going. Picture it: every ball of clay has six sticks coming out of it, two going to the balls of clay on top and bottom of it, two going to either side of it, and two going to the balls of clay in front and behind it. It’s like a bunch of cubes stacked on each other. There's a good model for the crystal structure of certain solids.

The crystal doesn't have to look like a bunch of cubes, but it can. The kind of pattern a crystal makes depends on the solid you are talking about. Copper, for example has a face centered cubic structure, which means all the copper atoms hang out with each other in a pattern that looks like a die where all the sides are fives. Diamonds make tetrahedrons!

Quick aside: Don’t get confused when I say crystals. I’m not talking about the sparkly things you hang on your rear view mirror. I’m talking about the repeating pattern that the atoms that in a solid make. Yes, some crystal structures can have a resulting macroscopic solid the looks crystalline. For example, jewels and gem stones with repeating patterns of sharp edges, facets, and glittering faces. Most crystalline solids, however, form a jumbled array of microcrystals called grains. The microcrystals are so tiny that the crystalline nature of the solid is not visible to the human eye, but it’s very apparent as a dominant feature of the solid's structure when magnified some 2500 times.

Anyway, glass is what’s called an amorphous solid. This means that it doesn’t have a crystalline structure. It’s a little more like a liquid because after the sand gets melted down and the molecules start moving around chaotically, glass makers then cool the molten glass rapidly before the molecules have time to organize themselves into a crystal pattern. The particles freeze in a random pattern. The way that the molecules bond with each other is all disorganized and crazy; it's no longer a stack of microscopic cubes.

Basically, as long as something is heated up really hot and then cooled rapidly before the molecules have time to arrange themselves in a crystal pattern, it’s an amorphous solid. Other amorphous solids include wax, rubber, certain kinds of clear candy, and plastics.

OK, So what makes it see-through? This is actually a two parter. Part one: Now that the crystals aren't neatly stacked, there are gaps and holes. It’s the difference between stacking your legos neatly in the box and just throwing them all in there. One of those ways is going to take up more room because there are spaces in between the blocks. Light can get through these gaps. That is, of course, if the light is of the right wavelength to fit through the hole. In the case of glass, visible light has a wavelength that seems to be comparable to many of the holes. That’s partially why the visible light can get through, or be transmitted.

One way of filtering certain types of light is by slowing down the cooling rate and allowing the glass to form crystal patterns. This can block out Ultra Violet light, for example. People that make sun glasses have this down. I’m pretty sure this is not why transparent aluminum is see-through, though. At least it's not the only reason.

So of course, there is more to being transparent than being an amorphous solid. Diamonds, for example, are transparent but they quite famously form crystal structures. One might argue that they are the ultimate crystal.

The transparent properties of a material are all about the band gap. This is where shit gets crazy...

We all learned about electrons living in their own shells, or orbitals, around the nucleus of an atom in high school, right? The distance an electron is away from the center of an atom (this is called it’s orbital radii) is determined by it’s energy. These orbitals are quantized energy states.

What does that mean?

The popular analogy here is to think of it like stair steps but I’m gonna make up my own.

We are used to this continuous way of life where I can stand on this side of the room, or that side of the room, or anywhere in between this side and that side of the room. But that doesn’t work in an electron's world. They follow different rules than us. You can either be on this side of the room or that side and there is NO in between. So an electron’s energy is quantized, it comes in packets where you get it all at once or it gets taken away all at once.

This is exactly what is happening when electrons hop up to higher or lower energy states.

How?

With photons! Photons are the particle that is associated with light. They are massless bundles of pure energy. If an electron absorbs a photon it gets all of it’s energy and if it ejects a photon it loses energy equal to the energy of the ejected photon. This will put it in a different orbital.

The energy of a photon depends on the wavelength of the light that it comes from. The bigger the wavelength, the smaller the energy.

Anyway…back to the band gap. Basically, it’s a difference between the energy levels of two groups of electrons. The two groups are called the conduction band and the valence band. A solid is a good conductor (meaning electrons have an easy time moving inside of it) IF AND ONLY IF the band gap is small. The valence electrons have every opportunity to hop up to a higher energy level…the next one up is so close, after all. That means the electrons are very likely to absorb an incoming photon and hop up into the conduction band. Once an electron is in the conduction band it’s free to cruise around and make TVs work and stuff. Current is the working name for electrons moving over the face of a solid, btw.

SO, the band gap is pretty small in things that are good conductors. Metals tend to be good conductors. Hm, metals also happen to be the opposite of transparent. They are reflective. Shiny. Including aluminum, it’s a great conductor with a very small band gap.

WHY SHINY: As I mentioned, all the photons that hit the metal get absorbed by the electrons because they have a really easy time jumping over the small band gap into the next energy level. Since light is an electromagnetic wave, the electric field of the light induces a current in the metal which ejects the incoming photons back out immediately and the surface appears reflective if the metal is smooth. If it’s not smooth the metal will have a dark appearance because all the incoming photons are getting absorbed instead of reflected or transmitted.

How cool is that? I love thinking about how the appearance of everything we can see is just the result of photons interacting with electrons and crystal structures of solids.

Ok, so that’s why things are shiny but why are they transparent?

If the band gap is large (as it is in insulators, the opposite of conductors) the electrons can’t absorb any photons because they don’t have the option of jumping to a higher energy state. That means the photons go right through the crystal structure of the solid with out being absorbed by electrons along the way.

The size of the band gap determines what kind of light will be transmitted and what will be reflected; Because remember, the band gap is an energy difference...energy of light is determined by it's wavelength...wavelength determines the type of light we are talking about. What kind of light will this object transmit and what will it reflect? Red? Blue? Infrared? X-rays? Depends on your band gap!

Maybe this is a stretch, but it just occurred to me: the atmosphere is made of stuff that is transparent to visible light but opaque to infrared light. That's the whole reason the green house effect exists. So, the molecules in the atmosphere must have a band gap with a smaller energy than visible light but the same size as infrared light. Or maybe you can't think of it like that because the atmosphere is a gas and not a solid. Who knows? Not me!

That was a lot of stuff. Good review session for me, for reals.

Ok, so how did they make aluminum, which should be shiny, as it is a metal and good conductor, the opposite of shiny? Also, does that mean that it no longer has conductive properties? This is what I needed to find out.

So, according to phys.org they used an extremely high powered and teeny tiny focused x-ray laser. (aside: you know laser is an acronym, right? Love it.) When I say teeny tiny I mean a twentieth of the diameter of a human hair. They managed to use this laser to kick out a core electron. That means, they removed an electron that is not a valence electron. So, it was somewhere deep inside the electron cloud and not an electron that usually gets messed with in regular chemical reactions. They were able to do this without disturbing the crystal structure of the aluminum.

How did that make it see-through? I can only assume that by removing a core electron the rest of the electrons switched around and ended up in an arrangement that gave it a much bigger band gap. Not sure.

I seem to have reached an impasse so I emailed my physics prof. Maybe she can explain it to me!

2 comments:

Nice write-up. It's been a while since modern physics and optics and this was a well written summation of some of the key concepts (with good analogies, I like to visualize things like that!) I'd be curious if you get any more good info from your prof... I suppose if they removed a core electron, the valence electrons might somehow settle into a lower energy state without disturbing the crystalline structure, thus increase the band gap. I can't really figure the details though. Interesting stuff!